Preheat the oven to 180°C (350°F) and grease a 9"x9" baking tin with butter or non stick cooking spray.
Beat the butter, brown sugar and sugar together.
Add in the eggs and vanilla extract and beat until smooth.
Add in the flour, baking soda and salt and mix. Finally add in 200g of the chopped Kinder Bueno, 150g of the Kinder chocolate and the chocolate chips if you're adding them and mix together.
Spread the cookie dough in an even layer and top with the remaining Kinder Bueno and Kinder chocolate and bake for 20-25 minutes. The top and sides will be golden and set but the middle will still be soft.
Allow to cool for around 10 minutes before removing from the pan and slicing.